This invention relates generally to processes for the decomposition of nitrates in solution. More particularly, it relates to a single-step process for the pressurized decomposition of ammonium nitrate and/or selected metal nitrates in aqueous solution.
Various industrial processes generate aqueous waste solutions of ammonium nitrate which pose problems with respect to release to the environment. For example, various chemical processes for the treatment or production of nuclear-reactor fuels generate such waste solutions, these solutions often containing radioactive solutes such as impurities or incompletely recovered compounds of nuclear-fuel metals (uranium, thorium, plutonium).
Preferably, the ammonium nitrate in such solutions would be decomposed to form environmentally acceptable waste products, any radioactive impurities or valuable fuel-metal compounds being recycled for recovery. However, practical rates of decomposition of ammonium nitrate are very difficult to achieve at atmospheric pressure. Furthermore, the decomposition of ammonium nitrate solutions under pressure has not been attractive hitherto because of the high temperatures and pressures required (e.g., 240.degree. C. and 500 psig) and because of the high corrosivity of the solutions at such temperatures.
Various de-nitration processes are described in U.S. Pat. No. 3,275,293, "Conversion of Fuel-Metal Nitrate Solutions," issued to P. A. Haas on Apr. 3, 1973. The patent describes in detail a single-step process for converting a fuel-metal nitrate to a fuel-metal oxide in pressurized, heated aqueous solution. In that process, a water-soluble nitrate-reducing agent selected from the group consisting of hydrazine and hydroxylated organic compounds consisting of a carbon, hydrogen, and oxygen is incorporated in the solution to be treated. The resulting solution is maintained at an elevated temperature and pressure to (a) decompose the nitrate to volatile products and (b) precipitate the fuel metal as fuel-metal oxide. A pressure-relief valve vents gases from the reaction zone whenever the operating pressure exceeds a preselected value corresponding to a preselected operating temperature. The oxide precipitate is recovered and dried to provide a powder which may be pelletized for use as a nuclear fuel.
Unfortunately, the above-described patented process is subject to certain limitations. Hydrazine is perhaps the most generally suitable reducing agent for use therein, but hydrazine generates some ammonium-nitrate waste, which is undesirable in many applications. On the other hand, hydroxylated organic reducing agents--e.g., methyl alcohol--do not give complete and reproducible reactions at temperatures below about 200.degree. C. At such temperatures, the process solutions are so corrosive that the reactor and various other system components need be composed of costly special materials rather than off-the-shelf materials.
The following United States patents relating to removal or decomposition of nitrates in solution have been noted: U.S. Pat. No. 3,008,904, "Processing Radioactive Waste," Dec. 29, 1959; U.S. Pat. No. 3,158,577, "Method of Treating Radioactive Waste," Nov. 24, 1964; U.S. Pat. No. 3,673,086, "Method of Removing Nitric Acid, Nitrate Ions, and Nitrite Ions Out of Aqueous Waste Solutions," June 27, 1972; U.S. Pat. No. 3,862,296, "Conversion Process for Waste Nitrogen-Containing Compounds," Jan. 21, 1975; and U.S. Pat. No. 3,962,114, "Method for Solidifying Liquid Radioactive Wastes," June 8, 1976. These patents contain no teaching or suggestion relating to the decomposition of nitrates in aqueous solutions also containing nitric acid, and nitromethane.